AirChat: Long-distance digital communications via radio

Last month, I was interviewed by NK News regarding the possibility of using inexpensive SDR dongles as a means for citizen journalists to receive and potentially send information across the North Korean border. Of course this is possible: digital communications over radio is becoming easier and more accessible all of the time.

While not yet as portable as an SDR dongle, Anonymous is developing a tool called AirChat which will allow long-distance communications via radio. AirChat borrows from Fldigi: a free application that decodes a variety of digital modes and has been used by amateur radio operators for years. Indeed, Fldigi is the same software you’ve used to decode broadcasts from VOA Radiogram and the new STF Radio.

Many thanks to the excellent Southgate ARC news site for this article about AirChat:

The International Business Times reports that the online hacktivist collective Anonymous has announced that it is working on a new tool called Airchat which could allow people to communicate without the need for a phone or an internet connection – it uses radio waves instead

Initially the data mode software AirChat used code from ‘minimodem’ and then from ‘soundmodem’ sources but they say after suggestions from radio amateurs involved in the Amateur Radio Emergency Service (ARES), they decided to make it modular to use the Fldigi software, a broadly deployed solution for use with ham radios.

They say “So far we have played interactive chess games with people at 180 miles away. we have shared pictures and established encrypted low bandwidth digital voice chats. We have 3D printed over distances of 80 miles and transmitted medical orders at distances of over 100 miles.”

So far Yaesu FT-897D’s amateur transceivers have been used and the developers are also looking at using low-cost ($40) VHF/UHF handheld transceivers.

If you do it right, which might not be easy, spread-spectrum on shortwave can go undetected – especially in the US: Use a a band near the MUF (maximum usable frequency). Put your gear into a depression so no potential eavesdropper can hear you over the ground wave.

Even a satellite would have quite some problem to detect you: Either it is geostationary and therefore 38.000 km away. I do not expect a satellite for that frequency range to have a good enough directional selectivity to pinpoint such a signal.

A low-orbit satellite on the other side wold be at least 300 km away and that for a minute or so at a time.

If they really insists on transmitting illegally they must do it in a way that cannot be detected easily. Conventional shortwave ham radio equipment like an FT-857 is definitely the wrong way to go. They should use ultra wideband technology that produces very low power intensities. SDR transceivers could do this.

Ultra wide band distributes the transmitter energy over a bandwidth that is much wider than the information would need. So if you listen to the signal with a conventional receiver you cannot detect it under the normal noise. This technology is widely used, e.g. by GPS. Rumors are that the military uses it extensively for the above purposes.

The key would be a spreading algorithm that every transmitter can generate locally like you can generate encryption keys. Only those who know this key can receive the signal.

This is probably somewhat outside the remit of exclusive SWLing, but I just came across these YouTube videos, which show a technique that seems achieve some of AirChat’s stated goals, all with off the shelf radio equipment and iOS apps:

Further to my last comment, I see an interesting convergence of tablets, SDR (especially USB dongles, see http://youtu.be/aLnNQOI-9CA ) and apps that encode/decode digimodes. I think general radio communication for tablets is not far off: think an HF modem like a 3G dongle in terms of its design and ergonomics but allowing global communication without any infrastructure.

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